1. Field of the Invention
The present invention relates to chemical-mechanical polishing (CMP) of semiconductor wafers. In particular, the present invention relates to an apparatus providing simultaneously polishing of multiple wafers.
2. Discussion of the Related Art
In integrated circuit manufacturing, CMP is widely used for planarizing the surface of the semiconductor wafer to allow multiple layers of conductors and dielectric to be formed.
In the prior art, CMP is achieved by pressing a semiconductor wafer against a polishing pad provided on a high-speed rotating table or a linear motion polishing belt. Typically, the semiconductor wafer is held by a wafer carrier, which also rotates. A slurry, typically including fine silicon oxide particles suspended in an alkaline solution, is provided as a chemically active abrasive.
In CMP using a rotational table, because each point on a semiconductor wafer experiences a polishing speed that depends, among other factors, the distance from the rotating table""s axis of rotation and its own speed of rotation, uniformity of polishing across the wafer is very difficult to achieve. Furthermore, because of the complex motion, the wear and tear on the polishing pad at different points of the rotating table are also non-uniform, also contributing to non-uniform polishing results.
Linear polishing eliminates some of the contributing factors of non-uniformity. However, because the contact surface areas at different points of the polishing pads are different, polishing non-uniformity is still difficult to achieve.
The present invention provides a method and an apparatus for accomplishing chemical-mechanical polishing of a wafer using a non-rotatory circular motion. This circular motion polishes every point on a wafer equally and also imparts equal wear and tear at every point of the polishing pad. Better uniformity than achievable in the prior art is therefore achieved.
According to one aspect of the present invention, an apparatus for polishing of a wafer includes (a) a wafer holder for holding the wafer so as to expose a surface of the wafer for polishing; (b) a polishing pad holder for holding a polishing pad against the exposed surface; and an actuator coupled to the polishing pad holder for driving the polishing pad in a non-rotatory circular motion so as to provide polishing action against the exposed surface of the wafer. In one embodiment, the apparatus multiple wafer holders are provided on a rotatable table, such that tasks related to chemical-mechanical polishing (CMP) of the wafer can be carried out simultaneously at stations around the rotatable table. In one embodiment of the present invention, the wafer holder includes a raised wall for containing a slurry used in polishing.
In one implementation, the actuator includes a motor which drives an off-center shaft to provide the non-rotatory circular motion. In that implementation, a linear bearing couples the polishing pad holder to the actuator, such that the off-center shaft, the linear bearing and the motor are enclosed in multiple connected chambers. In that arrangement, a pressurized air flow is provided to flow through the multiple connected chambers to effectuate cooling of the motor.
In one embodiment, the apparatus provides the polishing pad holder and the actuator in each of two polishing assemblies. In addition, a conditioning station is provided, so that when one of the polishing assemblies is positioned for polishing the wafer, the polishing pad in the other polishing assembly is positioned at the conditioning station for conditioning. A diamond plate on a rotatable platform is provided at the conditioning station. Conditioning is carried out by pressing the polishing pad against the rotating diamond plate.
In one implementation of the apparatus, an actuator is provided for rotating the wafer in the wafer holder during polishing.
According to another aspect of the present invention, the wafer holding includes an wafer edge extension ring which surrounds the wafer. The wafer edge extension ring has a surface flush with the surface of the wafer being polished, so that the edge of the wafer is xe2x80x9cextendedxe2x80x9d to the outer edge of the extension ring. Since polishing is carried out over the surfaces of the wafer and the wafer edge extension ring, edge effects are substantially eliminated.
In one implementation of the apparatus, the wafer is supported by a housing seated on a rotatable platform. The housing is provided an inlet into a recessed portion of the housing underneath the wafer holder, and a flexible seal is provided over the recessed portion of the housing in contact with a surface of the wafer holder. Under this arrangement, a chamber is formed under the surface of the wafer holder. A gas can then be introduced into the chamber through the inlet to allow a pressure to be applied against the wafer holder. The polishing pressure can then be controlled by adjusting the pressure on the flexible seal.
The present invention is better understood upon consideration of detailed description below and the accompanying drawings.